Conventionally, boilers provided in thermal power stations and a variety of large-scale boilers employing a fuel such as petroleum, coal, or fuel gas, waste incinerators, and similar apparatuses have been equipped with a flue gas NOx removal apparatus which contains a plurality of NOx removal catalyst layers.
The above employed NOx removal catalysts assume the form of honeycomb, plate, etc. During use, the catalytic performance of the catalysts is problematically deteriorated with elapse of time as a result of deposition, on the surface of the catalyst, of a substance which deteriorates the catalytic performance (hereinafter referred to as deteriorating substance) or through migration of the dissolved deteriorating substance into the catalyst.
Conventionally, the performance of the NOx removal catalysts has been managed by measuring NOx concentration and unreacted NH3 concentration on the inlet and outlet sides of respective catalysts. When a drop in total performance of a catalyst system is confirmed, old catalysts are replaced with new catalysts or regenerated catalysts in order of use age, and such replacement is carried out periodically.
Generally, NOx removal catalysts are very expensive. Thus, there has been proposed one approach for prolonging the service life of the NOx removal catalysts to as long a duration as possible by assessing the performance of each unit catalyst (see, for example, Patent Document 1).
In the aforementioned case, when the catalytic performance is evaluated by the percent contribution calculated on the basis of the NOx concentration, the catalyst(s) having actually deteriorated performance cannot be detected correctly. Thus, there has been proposed a performance assessment method which provides assessment results reflecting the actual state of the catalyst more accurately, the method including determining NH3 concentrations on the inlet and outlet sides of respective catalysts and taking into account the inlet mole ratio (i.e., inlet NH3/inlet NOx) (see, for example, Patent Document 2).
In an alternative performance evaluation method, instead of evaluating performance in an actual NOx removal apparatus, test pieces are cut out from an NOx removal catalyst, and the pieces are tested in a testing machine. For example, SV values based on catalyst amount and reacted gas amount, AV values based on catalyst surface area and amount of reacted gas, and other parameters may be obtained.
However, the aforementioned performance evaluation methods are developed without taking into account the flow of gas in the gas conduits, and overall catalytic performance of some NOx removal catalysts having a certain length cannot be fully evaluated.
Patent Document 1: Japanese Patent Publication (kokoku) No. 7-47108 (p. 2 to 3 and FIG. 1)
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2004-066228 (Claims and other sections)